1. Field of the Invention
The present invention relates to shuttle-type image printing devices which print characters and images on a print medium by scanning multiple printheads across the print medium. In particular, the invention provides for improved output from shuttle-type printing devices in which multiple printheads are disposed at a fixed distance from each other and wherein each printhead scans and prints over an assigned section of a print medium.
2. Description of the Related Art
Some conventional printing devices use full-line printheads, which are capable of simultaneously printing an entire line of data upon a print medium. Unfortunately, such printheads are quite expensive.
In contrast, serial printing devices operate by scanning a printhead across a print medium. The printhead forms images upon the print medium as it is scanned across. Such printheads are required to print only a small amount of data at any one time and are therefore generally less expensive than full-line printheads. Accordingly, serial printing is widely used in conventional printing devices.
Regarding color printing, several types of serial printing devices print color images by means of a print medium which itself generates color. Examples of such devices include a device in which heating elements on a thermal printhead heat special thermal paper, thereby generating color, and a device in which optical effects create color upon photo-sensitive paper.
On the other hand, various methods are used in which printheads transfer color ink onto print media. For example, in an impact printing method, ink ribbons contain liquid color ink which is transferred to a print medium when printing wires press the ribbons against the print medium. In thermal melt and sublimation transfer printing methods, heating elements on a thermal printhead heat solid ink on ink ribbon printheads and transfer the ink to a print medium. In an ink jet method, liquid ink is ejected onto a print medium.
Of the above examples, devices in which color ink is transferred onto print media are most widely used due to their ability to print color upon ordinary paper. Among these methods, ink jet printing has the advantages of low noise, lower operation cost, ease of miniaturization, ability to use ordinary paper, and ease of color printing. Hence, ink jet printing is widely used in various printing devices, such as printers and photocopiers.
In serial printing, a printhead can print on a relatively limited area at one time. This area is defined by printing elements, such as ink jet nozzles, located on the printhead. Accordingly, print speed is less than that of other printing methods, such as full-line or laser printing.
Many techniques have been introduced to attempt to increase the speed of serial printing. Examples include the use of a printhead having a wide printing swath (the width of an array of printing elements) and reduction of the scanning period by increasing carriage speed and printing frequency. Each technique, however, has accompanying limitations.
For example, a printhead having a wider printing swath is expensive because the precision required to manufacture such a printhead is not easily achieved by modern equipment. Moreover, wider printheads require larger print buffers, which are memory areas in which print data is temporarily stored.
With respect to methods in which heat is used to generate color on a print medium or to transfer ink to a print medium, a wider printhead generates more heat. The resultant higher temperature induces degradation or damage of printer components. Such degradation or damage must be prevented by some means.
In ink jet printing, liquid ink is propelled toward a print medium. Accordingly, a printhead having a wider printing swath causes more ink to be absorbed by a print medium and, as a result, the print medium cockles, or ripples. These ripples degrade print quality. No satisfactory methods to prevent such degradation have been proposed.
Where print speed is increased by increasing printing frequency, scanning speed must be increased correspondingly in order to maintain proper pixel density of image data. In this case, a larger load will be placed on a carriage motor. In addition, fast movements of the carriage shake the stored ink and thereby degrade print quality.
Japanese Laid-Open Patent Application Number 50-81437 and U.S. Pat. No. 4,272,771 disclose examples of methods to increase the print speed of serial image printing devices. According to these references, the left and right halves of each printed line are printed simultaneously. To achieve this feature, left and right printhead assemblies are provided, both of which are supported by a common carriage mechanism. Accordingly, print speed is approximately doubled over that of serial printing devices. Furthermore, these references suggest that further increases in print speed can be achieved by using more than two printhead assemblies or by printing in both the left and right scanning directions.
However, transverse and lateral registration between printheads becomes important in a printing device having multiple printheads which print on the same paper simultaneously. When the transverse registration is not adjusted correctly, there will be a transverse mismatch in the image printed by the left and right printheads. This mismatch is very noticeable at the boundary of the two areas printed by the left and right printheads. When the lateral registration is not properly adjusted, the two areas printed by the left and right printheads become separated or overlapped.
Therefore, ink jet nozzle adjustment for multiple printheads is necessary, not only for the above conventional example, but also for printing devices, such as a color printer, in which each of multiple printheads utilizes a different ink.
Bi-directional printing is another way of increasing print speed. In bi-directional printing, a serial printhead prints as it moves in each direction of its reciprocal scanning movement. Therefore, transverse and lateral printing positions corresponding to one of the reciprocal movements must match those of the opposite reciprocal movement.
In addition, in a structure where multiple printheads print on the same paper, ink density in a printing area assigned to one printhead may differ from that of assigned to another due to the difference in the characteristics of the printheads or other printer elements, such as inks or ink ribbons.
FIG. 1A and FIG. 1B illustrate this phenomenon. In FIG. 1A, two printheads, printhead 4A and printhead 4B, have printed within the section designated A and the section designated B, respectively. As shown, printhead 4B produces a more dense output than that of printhead 4A. The Figure illustrates the printing results for three printing duties, 25%, 50% and 100%. The Figure shows that, for each printing duty, the difference in print densities between section A and section B is very noticeable at the border between the two sections.
FIG. 1B illustrates similar printing results utilizing the same printheads while redefining section A and section B so as to add a small overlap between the two sections. Each printhead prints approximately half of the total print data in the overlapped printing area. Hence, the printing density of the overlapped area is greater than that of section A. However, the density is lower than that of section B. Therefore, in the case of FIG. 1B, the density differences are less noticeable than that shown in the above FIG. 1A, but are still obvious at both borders of the overlapped printing area. Accordingly, it is necessary to compensate for differences in print density caused by differences in output characteristics of utilized printheads.
Furthermore, in a printing device utilizing the above-described bi-directional printing method, density differences appear between bands (swathes) printed in one scanning direction and those printed in the other scanning direction, due to differences in printing characteristics in each direction.
Differences in printing characteristics arise because, in an ink jet printing device, ink jets propel satellite ink droplets in addition to main ink droplets. The relative location at which the satellite droplets land on a print medium with respect to that of the main droplets is different for one direction of the scanning motion than for the other. Hence, the area which is covered by ink differs in each direction. Therefore, print density needs to be compensated for with respect to the difference in the output characteristics in both directions.
In order to compensate for the above-described ink jet nozzle misalignments and density differences, one must initially determine the nature and degree of the ink jet nozzle misalignments and the density differences.
Conventionally, a measurement of these measurement objects relied on a visual judgment by a user or readout by optical sensors after printing test patterns. However, when a user makes adjustments using visual inspection and selections, problems occur because the adjustments turn out to be a burden on the user, or because a user does not make correct adjustments.
Accordingly, automatic measurement of measurement objects and subsequent adjustment is more desirable from the viewpoint of operability and reliability. However, extremely accurate sensors are needed to perform precise adjustments. In view of current technology, such accuracy is quite costly.